A leaky coaxial cable (hereinafter referred to as “LCX”) includes an inner conductor, an insulator, an outer conductor, and an outer sheath, and has heretofore been installed along the Shinkansen tracks (high speed railway line in Japan) and used for radio communications between trains and a ground terminal, or has been installed on subway premises or underground malls and used for fire or police radio communications with ground terminal as disclosed in Patent Document 1 to Patent Document 8. This LCX is provided with periodic slots in the outer conductor in order to leak electromagnetic energy from an inside of the coaxial cable to the outside of the cable.
Specifically, the outer conductor of the leaky coaxial cable is provided cyclically with multiple elongated-hole shaped slots for each constant cycle along a cable axis. Each of the slots is inclined by a certain angle relative to the cable axis. A leak electromagnetic field is formed by the leaky coaxial cable having the periodical slot arrays. Electric field components in a periaxial (near or around an axis) direction of the leak electromagnetic field can be approximately analyzed by approximating the slot arrays to magnetic current sources distributed on the axis in the axial direction, and then by calculating electromagnetic field formed by these magnetic current sources.
Here, in the LCX, when a pitch interval of the slot units coincides with a wavelength of an operating frequency, a resonant state is established. Accordingly, there is a problem that the operation of the LCX becomes compromised because part of electric power inputted to the LCX returns to a transmitter side, which makes it difficult to achieve broadband capability.    Patent Document 1: JP-A 5-121926    Patent Document 2: JP-A 6-69720    Patent Document 3: JP-A 7-131236    Patent Document 4: JP-A 9-83243    Patent Document 5: JP-A 9-35547    Patent Document 6: JP-A 10-145136    Patent Document 7: JP-A 10-276037    Patent Document 8: JP-A 2003-273641